This gets rid of an often-bogus diagnostic, and lets
at least the unlink test code recover reasonably when
it hits that brief window while another CPU has gotten
the complete() callback but hasn't yet resubmitted.



Return distinct code when unlinking an urb that's not linked.

This lets drivers handle this fault sanely, when they need to.
Gets rid of annoying non-error messages about drivers that unlink
in disconnect() even when the urb isn't linked.